As shown in FIG. 1, the heart 100 is a pump, has four chambers and is divided into a right side and a left side by a muscular wall called the septum 102. The two chambers at the top are called the right atrium 104 and the left atrium 106 and the two chambers at the bottom are called the right ventricle 108 and the left ventricle 110. The atria and ventricles work together, contracting and relaxing to pump blood out of the heart.
Oxygen-poor blood enters the top of the heart through the inferior and superior vena cava veins and flows into the right atrium 104 and passes through the tricuspid valve to the right ventricle 108. After the right ventricle 104 fills, it contracts and blood flows through the pulmonary valve to the lungs. Oxygen-rich blood from the lungs enters the left atrium 106 through the pulmonary vein and through the left atrium 106 to the left ventricle 110. The left ventricle pumps the blood into the aorta.
The heart also has an electrical system that includes a pacemaker to control the contraction of the heart chambers. Normal heart rhythm is termed sinus rhythm. During normal sinus rhythm, the heartbeat starts with a miniature electrical impulse in the sinoatrial (SA) node 116, also referred to as the heart's “natural pacemaker” located in the right atrium 104. The electrical signal spreads across the atria and via the atrioventricular (AV) node 112 to the ventricles. The AV node 112 is a small mass of specialized cardiac muscle fibers that receives impulses from the SA node 116 and directs them to the walls of the ventricles. The AV node 112 creates a brief delay (about one tenth of a second) in the impulse to allow the atria to contract and force blood into the ventricles and then spreads the impulses rapidly across the ventricles to make them contract. The ventricles are electrically isolated from the atria and electrical signals reach them via the AV node 112.
The AV node 112 connects to a group of fibers (the His-Purkinje system) in the ventricles that conducts the electrical signal. A bundle of His 114 connected to the AV node 112 transmits the electrical impulses from the AV node 112 to the distal His-Purkinje system which extends throughout the endocardium of the right and left ventricles 108, 110. The electrical impulses reach the cells of the ventricles, causing cardiac muscles in the ventricles to contract. The ventricles are the muscular part of the heart that actually pump the blood.
An electrocardiogram (ECG or EKG) is a graphic tracing of the variations in electric potential caused by the excitation of the heart muscle plotted along a time axis. The variations in electric potential are detected at the body surface through electrodes that are placed on different parts of the body (limbs, chest wall). The signals are amplified and recorded by the electrocardiograph. The electrocardiograph is an instrument for recording the changes of electrical potential. The ECG records the depolarization (stimulation) and repolarization (recovery) potentials generated by the atrial and ventricular myocardium.
FIG. 2 is a schematic illustration of an output from an electrocardiogram for a normal heart rhythm. The electrocardiogram shows the deflections resulting from atrial and ventricular activity. A typical electrocardiogram consists of a regular sequence of deflections (waves), labeled P, QRS, T and U. The first deflection (P) is due to excitation (contraction) of the atria. The QRS deflections are due to excitation (depolarization) of the ventricles. The T wave is due to recovery of the ventricles (repolarization). The U wave is a potential undulation of unknown origin immediately following the T wave. The amplitude of each of these components (deflections) is dependent on the orientation of the heart within the individual and the electrodes used to record the ECG.
The heart rate is the number of times the heart beats per minute which can be calculated by counting the average number of beats for a given duration (typically 15-30 seconds). The linear distance between neighboring peaks of simultaneous heart beats on an ECG corresponds to the time necessary for a single cardiac cycle (heart beat). As illustrated in FIG. 2, the linear distance (labeled “time”) is measured between the peaks of neighboring QRS complexes.
The distance between the R waves in a given ECG signal is variable. When an ECG is performed, it is common to measure the heart rate for several cardiac cycles to determine how consistently the heart beats. In addition to analyzing whether the interval between waves from consecutive cardiac cycles remain consistent, the individual that analyzes the ECG also looks for how fast the heart is beating, the consistent shape of each wave, and the normality of duration and configuration of each wave.
An arrhythmia is an irregularity from the normal rhythm of the heart. Atrial fibrilation is an arrhythmia that is characterized by a rapid irregular heartbeat. Atrial fibrillation (AF) is a common sustained arrhythmia in which the atria contract rapidly and irregularly in a chaotic manner due to multiple electrical signals firing at 400 to 600 beats per minute. The AV node 112 (FIG. 1) filters out most of the additional electrical signals. However, more electrical signals reach the ventricles 108, 110 (FIG. 1) than normal, resulting in the ventricles beating at rates of 110 to 180 beats per minute faster than normal resting heart rate which is between 60 and 80 beats per minute.
AF is not immediately life threatening, but the risk of stroke is increased because the quivering atria beat too rapidly to contract effectively and with time they enlarge, which can lead to blood clots forming within the atria. If a blood clot leaves the heart and lodges in the brain, a stroke results. Also, the rapid beating of the ventricles for prolonged periods can result in weakening them which can lead to heart failure.
Often the symptoms of atrial fibrillation occur infrequently and can only be detected by continuous monitoring over a long time period on an ambulatory subject using a small portable ECG recorder, called a Holter monitor (continuous ambulatory electrocardiograph monitor). Electrodes are taped to the chest and wires are connected to a portable battery-operated recorder.
Standard practice for using an ambulatory monitor requires electrode placement in known positions on the body in order to perform either visual analysis or analysis by a computer program using template matches on the recorded signal because the QRS complex differs dependent on position of the electrode. The analysis of each trace is dependent on the position of the electrode corresponding to the trace. The analysis involves comparing the trace with a stored template of a normal trace at the same position. Thus, the placement of the electrodes is critical to the analysis and is performed by a person who has received special training in the placement of the electrodes. The need for a person skilled in placement of the electrodes increases the cost of the ECG and limits the use of the test to those who have already exhibited symptoms.
The ECG signal may include isolated premature ventricular contractions (PVCs) which are abnormal heart contractions that originate in the ventricles. An uninterrupted series of ventricular detections indicates a possible ventricular arrhythmia which may be fatal. However, isolated premature ventricular contractions may occur in patients with or without heart disease. A PVC event results from irritated ectopi foci in the ventricular area of the heart which cause premature contractions of the ventricles. The premature contractions are independent of atrial depolarization. As the QRS complex occurs earlier with a premature ventricular contraction and has a wider morphology, inaccuracies in measurement of R-R intervals of the beats before and after the premature ventricular contraction are common.
Typically, PVCs are identified using correlation with a template that matches known signal morphology and requires assumptions about the placement of the leads. The identified PVCs can then be removed from the ECG signal or beat detections can be corrected to prevent confounding R-R interval variance which is an indicator used in atrial fibrillation monitoring. Typically, the removal of the identified PVCs is performed manually by a technician performing the verification. Alternatively, the template matching can be performed by a computer.